Method for manufacturing stainless steel product by nitrogen absorption treatment and stainless steel product produced by the method

ABSTRACT

A method for manufacturing a stainless steel product, which comprises contacting a bulk product of a ferritic stainless steel having been formed into a desired shape through melting with an inert gas containing nitrogen at 800° C. or higher, to thereby convert the structure of the whole or a part of the bulk product to austenite and produce an austenitic product or a product having a two-phase structure comprised of ferrite and austenite, respectively. The method allows the reduction of the manufacturing cost of a formed austenitic stainless steel product to a satisfactorily low level, which results in the manufacture of a stainless steel product having satisfactory characteristics in both of strength and corrosion resistance at a satisfactorily low cost.

TECHNICAL FIELD

This invention relates to a manufacturing process of a stainless steelproduct by nitrogen absorption treatment and to a stainless steelproduct thereby obtained. More particularly, this invention relates to amanufacturing process of a stainless steel product by nitrogenabsorption treatment, which can decrease production cost of austeniticstainless steel which is considered to be difficult to process, and canmanufacture a stainless steel product having fully satisfactoryproperties in both strength and corrosion resistance, and to a stainlesssteel product thereby obtained.

BACKGROUND ART

Austenitic stainless steel is a material which is difficult to processand its difficulty to work into a desired shape raises its productioncost. As austenitic stainless steel is outstanding in both corrosionresistance and strength, it is desired that its products having desiredshapes, including even complicated ones, be supplied at a low cost.

In the field of powder metallurgy, attempts have been made to havenitrogen absorbed at the time of sintering, but as large-scaledequipment is required for the manufacture of molded products by a methodof powder metallurgy, the products which can be thereby manufactured arelimited in size and shape. Moreover, products made by a method of powdermetallurgy have numerous holes known as pores, and they affect themechanical properties of the products and make their mechanicalreliability questionable.

No nitrogen absorption treatment has been adopted for bulk materialsmade by melting. The reason is that it has been common knowledge that ifa metallic material is held in a nitrogen atmosphere at a hightemperature for a long time, a microstructure is coarsened andmechanical properties are seriously lowered. In other words, a loweringin the mechanical reliability of bulk materials thus produced has beenstrongly feared.

This invention has been made under such circumstances and has as anobject to provide a method of manufacturing a stainless steel product bynitrogen absorption treatment, which can decrease production cost ofaustenitic stainless steel which is considered to be difficult toprocess, and can manufacture a stainless steel product having fullysatisfactory properties regarding both strength and corrosionresistance, and to provide a stainless steel product thereby obtained.

DISCLOSURE OF THE INVENTION

In order to attain the above object, this invention provides amanufacturing process of a stainless steel product by nitrogenabsorption treatment, characterized by bringing a bulk product offerritic stainless steel, which is made by melting, forming andmachining into a desired shape, into contact with an inert gascontaining nitrogen at or above 800° C. to austenitize the productcompletely, or to austenitize it partially and form a two-phasestructure composed of ferrite and austenite (claim 1).

This invention also provides a stainless steel product characterized byhaving a completely austenitic structure which is formed by addingnitrogen through contact with an inert gas containing nitrogen, to abulk product of ferritic stainless steel which is made by melting,forming and machining into a desired shape (claim 2).

Moreover, this invention provides a stainless steel productcharacterized by having a two-phase structure of ferrite and austenitewhich is formed by adding nitrogen through contact with an inert gascontaining nitrogen, to a bulk product of ferritic stainless steel whichis made by melting, forming and machining into a desired shape, so as toaustenitize it partially (claim 3).

The manufacturing process of a stainless steel product by nitrogenabsorption treatment according to this invention and the stainless steelproducts thereby obtained will now be described in further detail withreference to examples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph in place of a drawing, showing an ingot offerritic stainless steel (Fe—24 mass % Cr—2 mass % Mo) made by meltingin a vacuum arc melting furnace, and weighing 3.5 kg.

FIG. 2 is a photograph in place of a drawing, showing a round rod formedby hot and cold forging from the ingot shown in FIG. 1.

FIG. 3 is a photograph in place of a drawing, showing a plate formed byhot and cold forging from the ingot shown in FIG. 1.

FIG. 4 is a plan view of a round shape specimen for a tensile testformed from the round rod shown in FIG. 2.

FIGS. 5(a) and (b) show X-ray diffraction patterns of a test specimensubjected to nitrogen absorption treatment and a material equivalent tothe test specimen, but not subjected to nitrogen absorption treatment,respectively.

FIG. 6 is a correlation diagram showing balance between strength andductility of the test specimen, an existing alloy and the equivalentmaterial not subjected to nitrogen absorption treatment.

FIGS. 7(a), (b), (c) and (d) are polarization curves showing results ofpolarization tests conducted by immersing the test specimen, 316Lstainless steel and the material equivalent to the test specimen but notsubjected to nitrogen absorption treatment in a 0.9% NaCl solution, aPBS(−) solution, a Hanks' solution and an Eagle's MEM solution,respectively, and evaluating them for corrosion resistance.

FIGS. 8(a) and (b) are optical micrographs showing surfaces of the testspecimen and 316L stainless steel, respectively, as observed after thepolarization test in the Eagle's MEM solution.

BEST MODE FOR CARRYING OUT THE INVENTION

According to a method of this invention of manufacturing a stainlesssteel product by nitrogen absorption treatment, a bulk product offerritic stainless steel made by melting, forming and machining into adesired shape is brought into contact with an inert gas containingnitrogen at or above 800° C., so that it may be austenitized completely,or may be austenitized partially to have a two-phase structure formedfrom ferrite and austenite. A technique of bringing a bulk producthaving a desired shape into contact with an inert gas containingnitrogen at or above 800° C. belongs to a nitrogen absorption treatmentclassified as a solid-phase absorption method. Nitrogen is added to thewhole product or a part thereof by heating it to or above 800° C. in aninert gas atmosphere containing nitrogen. The method of this inventionof manufacturing a stainless steel product by nitrogen absorptiontreatment makes it possible to obtain a product having a desired shapeeasily, since the product to which nitrogen is added is made of ferriticstainless steel which is easier to work than austenitic stainless steel.The scale of equipment in the powder metallurgy method, limits toshaping and mechanical reliability of products is dissolved.

The stainless steel product of this invention which can be obtained asdescribed is completely austenitized, or is partially austenitized andhas a two-phase structure formed from ferrite and austenite. Therefore,the stainless steel product of this invention is outstanding in bothcorrosion resistance and strength and is advantageously an inexpensiveproduct, since its cost of processing is low, even if it may have acomplicated shape. The addition of at least about 0.5% by mass ofnitrogen to a bulk product of ferritic stainless steel is sufficient forachieving the results as mentioned above.

EXAMPLE

An ingot of ferritic stainless steel (Fe—24 mass % Cr—2 mass % Mo)weighing 3.5 kg, as shown in FIG. 1, was made by melting in a vacuum arcmelting furnace. The ingot was cut into four blocks each of whichmeasures 25 mm square by 110 mm, and the blocks were hot-forged at 1100°C. and cold-forged at ambient temperature into a round bar with 9 mm indiameter by 90 mm and a plate measuring 15 mm square by 1.5 mm inthickness, as shown in FIGS. 2 and 3, respectively. A round shapetensile test specimen with a planar shape as shown in FIG. 4 was formedby machining the round bar shown in FIG. 2. These two kinds of testspecimens were given nitrogen absorption treatment by a nitrogenabsorption furnace.

Each test specimen was placed on a mesh board of SUS304 stainless steel,degreased and cleansed with acetone, and inserted in a nitrogenabsorption portion of the nitrogen absorption furnace. The furnace wasevacuated by a rotary pump until 2 Pa. Then, an inert gas containingnitrogen was introduced into the nitrogen absorption portion at a rateof two liters per minute, the nitrogen absorption portion was heatedfrom room temperature to 1200° C. at a rate of 5° C. per minute and thetest specimen was kept in contact with nitrogen at 1200° C. for 24hours.

After the nitrogen absorption treatment described above, the testspecimen was quenched in iced water from 1200° C. After removal of scalefrom a surface by polishing, a microstructure was examined by using anX-ray diffractometer. A CuKα tube was used for X-ray diffraction and2θ/θ was varied from 40 to 90 degrees at a rate of one degree perminute. The X-ray diffraction pattern as obtained is shown in FIG. 5(a).For comparison, FIG. 5(b) shows the X-ray diffraction pattern of amaterial equivalent to the test specimen, but not given nitrogenabsorption treatment. As is obvious from comparison of FIGS. 5(a) and(b), the test specimen given nitrogen absorption treatment was made ofperfectly austenitic stainless steel. The amount of nitrogen additionwas about 0.9% by mass.

Then, a tensile test was conducted on the test specimen by using anInstron-type testing machine with a capacity of 100 kN at a crossheadspeed of 0.5 mm per minute. FIG. 6 shows balance found between strengthand ductility of the test specimen which had been given nitrogenabsorption treatment, of the material equivalent to the test specimenwhich had not been given nitrogen absorption treatment and of aconventional alloy. As is clear from FIG. 6, the nitrogen absorptiontreatment realized a better balance between strength and ductility ascompared with the conventional alloy and the material equivalent to thetest specimen which had not been given nitrogen absorption treatment.The effectiveness of nitrogen absorption treatment was thus confirmed.

The test specimen was also evaluated for corrosion resistance.

The test specimen, 316L stainless steel and material equivalent to thetest specimen which had not been given nitrogen absorption treatmentwere polarized in a 0.9% NaCl solution, a PBS(−) solution, a Hanks'solution and an Eagle's MEM solution which had been prepared at 37° C.and deaerated with bubbling nitrogen gas. FIGS. 7(a) to (d) are thepolarization curves obtained by polarization tests conducted forevaluation on corrosion resistance. As is confirmed by FIGS. 7(a) to(d), the test specimen showed higher corrosion resistance in all of thetest solutions than the 316L stainless steel and the material equivalentto the test specimen not given nitrogen absorption treatment, and it didnot have any pitting corrosion, as is obvious from FIG. 8(a). Pittingcorrosion was found in 316L stainless steel, as shown in FIG. 8(b).

The method of this invention of manufacturing a stainless steel productby nitrogen absorption treatment and the stainless steel product therebyobtained overcome the difficulty in processing austenitic stainlesssteel and make it possible for even a product having a complicated shapeto be obtained at a low cost and have satisfactorily high levels ofstrength and corrosion resistance.

This invention is, of course, not limited by the mode or example asdescribed above. It is needless to say that variations or modificationsmay be made in details including the composition of stainless steel, theshape and size of the product and the conditions of nitrogen absorptiontreatment.

INDUSTRIAL APPLICABILITY

As is obvious from the detailed description above, this inventionprovides a stainless steel product formed from austenitic stainlesssteel which is considered to be difficult to process, has a desiredshape, can be processed at a satisfactorily low production cost and hasfully satisfactory properties with regard to both strength and corrosionresistance, and a method of manufacturing the same.

1. A manufacturing process of a stainless steel product by nitrogenabsorption treatment, comprising the steps of bringing a bulk product offerritic stainless steel made by melting, forming and machining into adesired shape, into contact with an inert gas containing nitrogen at orabove 800° C., and austenitizing the product completely.
 2. A stainlesssteel product having a completely austenitic structure formed by addingnitrogen through contact with an inert gas containing nitrogen, to abulk product of ferritic stainless steel made by melting, forming andmachining into a desired shape.
 3. (canceled)